1. Field of the Invention
The present invention relates to a pulse tube refrigerator and, more particularly, to a structure of a pulse tube refrigerator having an improved refrigerating efficiency.
2. Related Art
A conventional pulse tube refrigerator is disclosed on pp. 35 of Summary of 55th Symposium of Association of Low Temperature Engineering/Superconduction, as held in autumn, 1996. This pulse tube refrigerator will be described with reference to FIGS. 20 to 22.
In FIG. 20, a pulse tube refrigerator 111 includes a regenerator 1 having a cold end 1a and a hot end 1b; a cold head 2 connecting to the cold end 1a of the regenerator 1; a pulse tube 3 having a cold end 3a and a hot end 3b and connected at its cold end 3a to the cold head 2; a pressure fluctuation source connected to the hot end 1b of the regenerator 1; and a buffer 5 connected to the hot end 3b of the pulse tube 3 through an orifice 4. The pressure fluctuation source includes a compressor 10; a high-pressure control valve 11 connected to the outlet port of the compressor 10 through a high-pressure passage 18; a low-pressure control valve 12 connected to the inlet port of the compressor 10 through a low-pressure passage 19; and a connection passage connecting the high-pressure control valve 11 and the low-pressure control valve 12 to the hot end 1b of the regenerator 1.
The operation of the pulse tube refrigerator thus constructed will be described with reference to FIGS. 21 and 22. Here, FIG. 21 is a graph illustrating both the controlled states (of which the opened states are indicated by thick lines and the closed states are indicated by thin lines) of the high-pressure control valve 11 and the low-pressure control valve 12 over time, and the pressure states of the working fluid in the buffer 5 and the pulse tube 3 over time. FIG. 22 is an equivalent PV diagram illustrating the relation between the displacement and the pressure of the working fluid in the vicinity of the cold end 3a of the pulse tube 3.
As seen from FIG. 21, the operation states of the pulse tube refrigerator and the corresponding states of the internal working fluid may be divided in terms of the time into the following six Steps a to f:
(1) Step a (First Half Step of Compression)
The state in which the high-pressure and low-pressure control valves 11 and 12 are kept closed by closing the low-pressure control valve 12. In this state, the working fluid in the buffer 5 flows into the pulse tube 3 through the orifice 4 so that the pressure in the pulse tube 3 rises to the buffer pressure.
(2) Step b (Second Half Step of Compression)
The state in which the high-pressure control valve 11 is opened when the pressure in the pulse tube 3 rises from the minimum pressure to the buffer pressure. In this state, the high-pressure passage 18 and the pulse tube 3 come into communication so that the pressure in the pulse tube 3 rises from the buffer pressure to the maximum pressure.
(3) Step c (Transfer Step to High Pressure)
The Step in which the high-pressure control valve 11 is kept open. In this state, the working fluid in the pulse tube 3 continuously flows out to the buffer 5 through the orifice 4, and the working fluid from the compressor 10 flows into the regenerator 1 via the high-pressure control valve 11, and into the pulse tube 3 while being cooled in the regenerator 1.
(4) Step d (First Half Step of Expansion)
The state in which the high-pressure and low-pressure control valves 11 and 12 are kept closed by closing the high-pressure control valve 11. In this state, the working fluid in the pulse tube 3 flows into the buffer 5 through the orifice 4 so that the pressure in the pulse tube 3 drops from the maximum pressure to the buffer pressure. As a result of this pressure drop, the working fluid in the pulse tube 3 adiabatically expands to lower its temperature.
(5) Step e (Second Half Step of Expansion)
The Step in which the low-pressure control valve 12 is opened when the pressure in the pulse tube 3 falls from the maximum pressure to the buffer pressure. In this state, the low-pressure passage 19 and the pulse tube 3 come into communication so that the pressure in the pulse tube 3 drops from the buffer pressure to the minimum pressure. As a result, the working fluid in the pulse tube 3 further adiabatically expands to lower its temperature.
(6) Step f (Low-pressure Transfer Step)
The state in which the low-pressure control valve 12 is kept open. In this state, the working fluid in the buffer 5 continuously flows into the pulse tube 3 through the orifice 4, and the cold working fluid in the pulse tube 3 cools the cold head 2 and the regenerator 1 and flows out from the low-pressure control valve 12 to the compressor 10.
The foregoing Steps a to f comprise one cycle and are repeated to establish an extremely low temperature in the cold head 2.
The conventional system thus far described is characterized by providing Step a and Step d in the running cycle of the pulse tube refrigerator. The equivalent PV diagram of the case, in which Steps a and d are omitted and in which the high-pressure and low-pressure control valves 11 and 12 are alternately opened without any standby time, is illustrated by dotted lines in FIG. 22. On the other hand, the equivalent PV diagram of the case having Steps a and d is illustrated by solid lines in FIG. 22. It is apparent from the comparison between the equivalent PV diagrams of the two cases that the case with Steps a and d has a larger area for the section enclosed by the PV diagram. This area determines the upper limit of the refrigerating output of the regenerator, so that the refrigerating capacity can be enhanced without increasing a heat loss, due to the displacement by enlarging the area while retaining the magnitude of the displacement of the working fluid.
FIG. 23 is a schematic diagram showing another conventional pulse tube refrigerator. This pulse tube refrigerator 112 is constructed by connecting the hot end 3b of the pulse tube 3 and the buffer 7 via a buffer side control valve 6, but the remaining construction is identical to that of the pulse tube refrigerator 111 shown in FIG. 19. FIG. 24 is a graph illustrating both the controlled states (of which the opened states are indicated by thick lines and the closed states are indicated by thin lines) of the high-pressure control valve 11, the low-pressure control valve 12 and the buffer side control valve 6 over time when the pulse tube refrigerator of FIG. 23 is operating, and the pressure states of the working fluid in the buffer 7 and the pulse tube 3 over time. With reference to FIG. 24, the operation characteristics of this pulse tube refrigerator 112 will be described, stressing the control actions of the buffer side control valve 6.
(1) At Step a (First Half Step of Compression), the buffer side control valve 6 is opened to connect the pulse tube 3 and the buffer 7 so as to raise the pressure in the pulse tube 3 from the minimum pressure to the buffer pressure.
(2) At Step b (Second Half Step of Compression), the pressure in the pulse tube 3 has already risen to the buffer pressure and is raised to the maximum by closing the buffer side control valve 6 and by opening the high-pressure control valve 11.
(3) At Step c (Transfer Step to High Pressure), the buffer side control valve 6 is opened to transfer working fluid in the pulse tube 3 under a high pressure to the buffer 7. At this time, the working fluid flows from the compressor 10 into the regenerator 1 via the high-pressure control valve 11, and into the pulse tube 3 while being cooled by the regenerator 1.
(4) At Step d (First Half Step of Expansion), the high-pressure control valve 11 is closed to lower the pressure in the pulse tube 3 to the buffer pressure. As a result of this pressure drop, the working fluid in the pulse tube 3 adiabatically expands to lower its temperature.
(5) At Step e (Second Half Step of Expansion), the pressure in the pulse tube 3 has already dropped to the buffer pressure and is lowered to the minimum pressure by closing the buffer side control valve 6 and by opening the low-pressure control valve 12. As a result, the working fluid in the pulse tube 3 further adiabatically expands to lower its temperature.
(6) At Step f (Low-pressure Transfer Step), the buffer side control valve 6 is opened to transfer the working fluid in the buffer 7 to the pulse tube 3. At this time, the working fluid in the pulse tube 3 cools the cold head 2 and the regenerator 1 and further flows out from the low-pressure control valve 12 to the compressor 10.
The foregoing Steps a to f comprise one cycle and are repeated to establish an extremely low temperature in the cold head 2. The equivalent PV diagram of the working fluid in the run of the pulse tube refrigerator 112 thus far described is identical to that indicated by the solid lines in FIG. 22.
For an efficient operation of the pulse tube refrigerator, it is necessary to provide a sufficient time period to the high-pressure transfer Step (i.e., Step c) and the low-pressure transfer Step (i.e., Step f) of the working fluid. This is because the flow rate of the working fluid through the regenerator at the high-pressure transfer Step and the low-pressure transfer Step is higher than that at the remaining Steps, so that a longer time period has to be taken for reducing the heat loss at the regenerator.
In view of the conditions for making such pulse tube refrigerator efficient, here will be examined the running operations of the pulse tube refrigerator 111. In this pulse tube refrigerator, at Steps a and b, the communication between the working fluid in the pulse tube and the working fluid in the buffer is via the orifice 4 so that a relatively long time period is required for raising or lowering the pressure in the pulse tube to the buffer pressure. If the operation of the pulse tube refrigerator is to be realized within a limited cycle time period, therefore, the time period is mostly taken for Steps a and d so that Steps c and f have to be finished within a relatively short time period. This raises a problem that the heat loss in the regenerator is increased to make it impossible to run the pulse tube refrigerator efficiently. In the pulse tube refrigerator 112, on the other hand, the communication between the working fluid in the pulse tube and the working fluid in the buffer is made through the buffer side control valve 6. At Steps a and d, therefore, the pressure in the pulse tube is quickly raised or lowered to the buffer pressure, and the time periods for Steps c and f also have to be shortened. This is because the communication between the working fluid in the pulse tube and the working fluid in the buffer is made through the control valve so that the longer time period for Steps c and f make the displacement of the working fluid so large as to increase the heat loss due to the displacement. In short, the pulse tube refrigerator 112 must equalize the time periods for Steps a and d and Steps c and f. When a buffer side control valve having a small opening is used to extend the time period for Steps c and f, on the contrary, the time period for Steps a and d is made as long as that of the pulse tube refrigerator 111. Thus, the efficiency of the conventional pulse tube refrigerators is limited no matter what construction and running operation might be taken.